This invention relates to cardiac pacers, and, more particularly, implantable cardiac pacers adapted to be programmed by the use of externally generated signals for changing operating parameters of the pacer. This application relates to U.S. Pat. No. 4,124,031, assigned to the same assignee, which covers an implantable programmable pacer incorporating a logic system for receiving magnetic signals from a simple magnet source, which patent is incorporated herein by reference.
With new improved power sources now being used in the pacer industry, such as the lithium battery, as well as advanced techniques for providing long life reliability and low current drain in more complex circuits, programmable pacers are coming into increasing use. Such use to date of programmable pacers has indicated what the most important design features are. Of course, for any implanted biomedical device, it must be reliable. Reliability in turn is tied to simplicity of design and low power operation. Another important design consideration is that the pacer be immune from being accidentally programmed by unwanted external signals which may be in the environment of the patient. At the same time, the physician who treats the patient desires to have a pacer which can be easily and simply programmed, as by an automatic transmitter which is set by simply pushing several buttons, and without requiring any complicated procedure on behalf of the physician. Also, and particularly in view of the fact that the pacer may be implanted for a lifetime of perhaps up to 15 years, it is important that the pacer have a fallback or "fail-safe" means of being programmed in the event that the automatic transmitter is not available when neded.
A programming system for an implanted pacer has been provided, as disclosed in U.S. Pat. No. 4,124,031, which utilizes a simple magnet providing a constant magnetic field which is capable of penetrating the sealed pacer housing. The pacer is programmed when the physician places the magnet in the vicinity of the patient's implanted pacer for time periods which correlate with the patient's heartbeat in accordance with a predetermined program arrangement. The externally applied magnetic field activates a reed switch, the closing and opening of which in accordance with the predetermined program arrangement is detected, so as to initiate a change of a given pacer parameter in accordance with the predetermined program. This arrangement provides essentially the safest form of programming in terms of availability, since simple magnets are universally available, are found in all parts of the world, and can be expected to be found indefinitely into the future.
While simple magnet programming provides the advantages as set forth above, many physicians will not want to use a simple magnet because this would require that they follow a predetermined specific procedure in order to achieve proper programming. Physicians understandably in many instances would prefer an automatic external programmer, or transmitter, whereby the desired parameter is set in, as in a pushbutton keyboard, and then the program-encoded signals are transmitted automatically to the pacer. For the physician who will always be in his office or otherwise at a location where the programmer-transmitter is always available, he will certainly prefer the automatic transmitter as opposed to the manual procedure using a simple magnet.
Thus, there are good arguments in favor both of a programmable pacer adapted to be programmed by a simple magnet, and of an automatic transmitter. Ideally, the best solution would be to have both arrangements. However, the circuitry within the pacer for decoding programming signals is inherently complex, adding to the expense of the device. Extra circuitry also requires consumption of power and contributes to energy depletion of the battery source and thus contributes to a shortened pacer lifetime. In terms of reliability, the use of parallel programming systems would aid in ensuring that the patient can always be reprogrammed in one way or the other. However, many practicalities of the pacer design would dictate against parallel programming systems. For example, it is desirable to contain the pacer electronics on one or at most two chips, and a design that would mean adding an extra chip simply would not be acceptable.
The objectives of this invention are to overcome the disadvantages of the prior art programmable pacers. Since each prior art programmable pacer utilizes its own programming arrangement, i.e., own form of encoding the transmitted signals, the patient who carries such a programming pacer can be reprogrammed only with the specific programmer designed for use with the model pacer being worn. For this reason there is no reliable form of backup transmitter for providing programming in the event that the specific automatic programmer is not available. On the other hand, the pacer which has the capacity for programming one or more operating parameters by simple manipulation of a magnet carries the disadvantage of being vulnerable to extraneous signals, such that it is not secure, and even if made secure is in disfavor compared to automatic pacers because of the physician's desire for an automatic transmitter which obviates the physician's having to master the programming procedure. These disadvantages of the prior art are overcome by applicant's invention, wherein there is provided a pacer adapted to be programmed either by slow program signals (provided by a simple magnet) or fast program signals (provided by an automatic transmitter). The pacer of this invention provides additional advantages of reliability and cost reduction, as well as reduced power drain, by providing a common logic circuit, or program decoding circuit, which is compatible for accepting and analyzing both the fast and slow program signals. There is thus provided a pacer which overcomes the disadvantages of prior art pacers without requiring any significant additional complexity, being reliably programmable by an automatic external programmer or by the fail-safe backup method of using a simple magnet to provide the program signals. For decoding the fast program signals, the pacer utilizes a unique circuit for detecting the envelope of a modulated magnetic signal, the circuit providing both an envelope detector and a sharp bandpass filter characteristic.